With the increase of Internet traffic, an optical communication system in a trunk feeder system needs larger capacity. Meanwhile, as a bit rate per wavelength increases, degradation of information quality may become very severe due to chromatic dispersion, polarization mode dispersion, and various nonlinear waveform distortion on a transmission path.
Compared with incoherent technology, digital coherent receiving technology has the following advantages. An optical signal to noise ratio (OSNR) gain of about 3 dB can be achieved; channel variance can be dealt with and cost can be reduced by conveniently using electronic equilibrium technology; transmission capacity can be improved by using more efficient modulation technology and polarization multiplexing technology. Herein, linear distortion of optical signal, for example, chromatic dispersion (CD), polarization mode dispersion (PMD) and the like, can be almost completely compensated by using the electronic equilibrium technology. Therefore, the digital coherent technology is regarded as a key technology in a high-speed optical communication system.
FIG. 1 is a block diagram illustrating signal processing of a typical digital coherent receiver. As shown in FIG. 1, a procedure for performing signal process by the digital coherent receiver includes: an optical signal is divided into two mutually orthogonal polarized optical signals by a polarizing beam splitter (PBS) 101; the polarized optical signals outputted by the PBS 101 is frequency mixed with a local oscillator optical signal via a 90° photomixer 102; the frequency-mixed optical signal is converted to a baseband electric signal through a balanced photodetector (PD) 103; and the photo-electrically converted electric signal is converted to a digital signal by an analog-to-digital converter (ADC) 104, and then the digital signal converted by the ADC may be processed by means of a universal digital signal process technology.
The digital signal converted by the ADC may be processed using the universal digital signal process technology through following steps in sequence: a skew compensation module 105 performs skew compensation process, a DC (direct current) removing/IQ (In-phase/Quadrature) mismatch compensation module 106 performs DC removing/IQ mismatch compensation process, a chromatic dispersion compensation module 107 performs chromatic dispersion compensation process, a clock recovery module 108 performs clock recovery process, an adaptive equalization module 109 performs adaptive equalization process, a carrier synchronization module 110 performs carrier synchronization process, and a judgment detection module 111 performs judgment detection process.
A chromatic dispersion value generally is relatively large, thus compensation of chromatic dispersion and polarization mode dispersion generally is completed in two parts. First of all, the chromatic dispersion is compensated, an equalizer here generally is unable to use a standard adaptive algorithm for coefficient updating. For example, in order to compensate 40000 ps/nm chromatic dispersion, the number of taps of a filter needs to reach several hundreds or even above one thousand. Generally Fast Fourier Transform is used for fast frequency domain convolution, and a chromatic dispersion estimation module provides the chromatic dispersion compensation module 107 with a chromatic dispersion value to be compensated.
Then, residual compensation of chromatic dispersion and polarization mode dispersion is implemented by the adaptive equalization module 109, specifically, by a finite impulse response (FIR) butterfly equalizer. An FIR butterfly filter adopts an adaptive algorithm to update a coefficient so as to track and compensate the polarization mode dispersion dynamically changing with time. A function of the FIR butterfly equalizer is to implement polarization demultiplexing. The FIR butterfly equalizer plays a role in equalization, matched filtering and sampling position adjustment. When a variation range of a sampling position is too large, or a sampling frequency offset exists so that a sampling phase variation range is beyond an adjustment range of the FIR butterfly adaptive equalizer, the FIR butterfly adaptive equalizer may be unable to work properly. Therefore, a clock recovery module 108 needs to be placed prior to the FIR butterfly equalizer.
The clock recovery module 108 estimates a sampling time error of an input symbol, and performs interpolation adjustment on the sampling time of the symbol, or adjusts an ADC sampling frequency through a voltage-controlled oscillator (VCO) to ensure supply of a stable symbol sampling phase. When the interpolation adjustment is performed on the sampling time of the symbol or the ADC sampling frequency is adjusted through the VCO, it is required that a phase discriminator of the clock recovery module 108 should tolerate signal distortion to a certain degree. A conventional phase discriminator, however, generally can only tolerate a very small chromatic dispersion value. Therefore, it is required that the chromatic dispersion compensation module 107 should accurately perform chromatic dispersion compensation. For this purpose, the chromatic dispersion estimation module needs to provide an accurate chromatic dispersion value to be compensated, in other words, the chromatic dispersion value to be compensated needs to be measured at a high precision.
At present, there is no technical scheme which can measure a chromatic dispersion value to be compensated at a high precision.